1. Field of the Invention
This invention relates generally to the design of amplifiers and, more particularly, to the design of efficient, low-noise, high-speed amplifiers.
2. Description of the Related Art
Electronic amplifiers are used for increasing the power and/or amplitude of various specified signals. Most amplifiers operate by sinking current from a power supply, and controlling the output signal to match the shape of the input signal, but having a higher amplitude. Amplifiers are typically specified according to their input and output characteristics. One of the main characteristics of an amplifier is its gain, which relates the magnitude of the output signal to the magnitude of the input signal. The gain may be specified as the ratio of the output voltage and the input voltage, or the ratio of the output power and the input power. The gain relationship is oftentimes expressed as the transfer function of the amplifier. In most cases, the transfer function of an amplifier is expected to be linear, that is the gain is expected to be constant for any combination of input and output signals. While linear amplifiers respond to different frequency components independently, and do not generate harmonic distortion, nonlinear amplifiers are oftentimes affected by distortion. Overall, if the transfer function or gain is not linear, the output signal may become distorted. There are many classifications addressing different amplifier design considerations, oftentimes defining particular relationships between the design parameters and the objectives of a given circuit. Various power amplifier circuit (output stage) classifications exist for analog designs (class A, B, AB and C for example), and for switching designs (class D and E for, example) based upon the conduction angle or angle of flow, Θ, of the input signal through the amplifying device—that is, the portion of the input signal cycle during which the amplifying device is conducting. The conduction angle is closely related to the amplifier power efficiency, and the image of the conduction angle may be derived from amplifying a sinusoidal signal (e.g. if the device is always on, Θ=360°.) Amplifier design typically requires a compromise between numerous factors, such as cost, power consumption, device imperfections, and a large number of performance specifications.
Many systems, including audio systems, measurement and data acquisition (DAQ) systems, some of which are PC-based, plug-in boards, radio frequency transmission systems, and control systems make use of amplifiers. For example, in a measurement or data acquisition process, analog signals are received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. Then, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a DUT. Oftentimes however, the received signal is small relative to the dynamic range that is typical of ADCs. That is, the measured signal may have a small dynamic range, for example on the order of tens of mV in some systems. Therefore it is oftentimes required to further process the measured signal in order to match the dynamic range of ADCs. To achieve this, the measurement instruments may include switchable amplifiers to scale the measured signal to a level appropriate for the ADC or RMS-to-DC converter used in the measurement.
A power amplifier is typically considered to be the last amplifier in a transmission chain (the output stage), representing the amplifier stage for which power efficiency is a most important factor. The efficiency considerations for power amplifiers resulted in the definitions for a variety of different classes of power amplifiers, based at least in part on the biasing of the output transistors. As mentioned above, power amplifiers are classified as A, B, AB and C for analog designs, and class D and E for switching designs, among others, with additional classes including class G, for example. Class AB and G amplifiers are typically fast and quiet, that is, they produce a mostly noise-free output signal. Class D amplifiers are efficient but are also typically slow and noisy, that is, they produce an output signal that is not noise-free. It is possible to design low-noise, high-speed, and high-efficiency amplifiers by attempting to combine the best features of both topologies. However, the design of such amplifiers at present time is complex, and yields only partially successful results with respect to the goal of obtaining the best possible combination of the best features of the two topologies.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.